1. Field of the Invention
This invention relates to pressure sensitive adhesive (PSA) compositions prepared from novel anionic block copolymers of aromatic vinyl compounds and conjugated dienes. The invention further relates to die-cuttable labels and decals formed using the pressure sensitive adhesive composition.
2. Background of the Art
Pressure sensitive adhesives comprising styrenic block copolymers are well known.
The block structure of the polymer gives adhesives having high shear and peel strengths and high upper service temperatures. However, the high strength of such adhesives is not suitable for all applications. For instance, formulators would like the ability to adjust the cohesive strength of an adhesive based on a block copolymer to improve the die-cutability of PSA labels or to achieve a cohesive failure mode in a peel test.
Traditional approaches to improve the die-cutability of block copolymer based adhesives have been to reduce the cohesive strength by incorporation of diblock copolymer as taught in U.S. Pat. No. 4,096,203. Other approaches are to include low molecular weight, liquid rubbers as taught in U.S. Pat. No. 5,274,036. These approaches do serve to reduce the cohesive strength of the adhesive formulations. However, with the use of conventional block copolymer compositions, the high temperature performance is deleteriously reduced as the cohesive strength is lessened.
We have found that certain new and novel polymers will allow the preparation of improved pressure sensitive adhesive compositions. In particular, adhesives containing the presently claimed high molecular weight, low coupled (i.e., high diblock fraction) polymer composition maintain their high temperature performance while providing adhesives which fail cohesively. This heretofore unattainable balance of properties allows the variation of peel strength, where the peel fails cohesively, independently of the high temperature performance of the adhesive. This is particularly useful in the construction of die-cuttable adhesives. In this application, the adhesive must have low cohesive strength so it can be cut into the desired shape and the unwanted matrix removed, but it must also maintain its high temperature performance to prevent adhesive bleed into a label facestock paper. The present invention addresses this need.
In addition, the pressure sensitive adhesive composition of the present invention may comprise ester-type plasticizers typically used as PVC plasticizers. This behavior is unexpected for adhesives based on styrenic block copolymers since the ester-type plasticizers generally segregate into the styrenic endblocks and destroy the strength of the adhesive. Surprisingly, we have found the pressure sensitive adhesive compositions of the present invention able to tolerate relatively high levels of ester-type plasticizers. This makes the invention further useful as an adhesive for PVC labels and decals.